three-terminal semiconductor device in which input signal (voltage or current depending on the type of transistor) controls output current; the most important semiconductor device; performs switching and amplifying functions; early on replaced bulky and inefficient vacuum triode in electronic circuits; invention of transistor (Schockley, Brattain and Bardeen, 1947) triggered electronic revolution after the World Word II; can operate as a discrete device or a building cell of integrated circuits; numerous kinds of transistors can be distinguished based on their design and principles of operation; there are two types of transistors: unipolar (field-effect transistor, or FET) and bipolar.